how long has it been since it had a new carby kit? a flat spot could be anything and you should check everything in the process of getting rid of it, i.e. plugs and leads, rotor button and cap, fuel filter, air filter etc. then check timing. once you have done all that, move onto the carb.
check that your 4 fuel bowl screws are nipped up. and just nip them up dont over tighten cos you can bend the alloy.
get a felt pen and put a dot on either side of the 'line' in your idle mixture screws, do them up till they bottom out, carefully counting how many turns it took then make a note of it and unscrew them all the way out. get some very fine grit paper, (1500 or 2000) and very gently roll the idle screw over the sandpaper to get rid of any crap on it and make them shiny again DO NOT SAND THE **** OUT OF THEM, you dont wanna change their shape. blow some carby cleaner into the hole they came out of then refit the screws.
you got a vacuum guage? if so, you know how to use it? if not you can try and turn the idle screws in 1/16th of a turn and see if it increases you idle slightly. that is if it is running too rich. a lot of "i know about cars" people will say your engine is running rich cos' of the smell, but they dont realise that unleaded fuel has a really strong odour and we dont have cats to get rid of the smell, so they are often wrong. you will know if its running rich or lean by the colour of your spark plugs (which you checked before moving onto the carby).
so yeah, anyways. holley flat spots are most often caused by poor idle mixture settings. once you hit the 'off idle transfer slot' if you have too much mixture in your idle circuit, then the tranfer slot overpowers everything and the engine hesitates. if you havent got a vac guage you can try turning the idle mixtures a teeeensie bit and taking the car for a fang.
sorry for the lecture but im gonna continue cos' i like the sound of my own voice.
there are a few main parts to a holleys carburation.
1. idle circuit (idle mixture screws)
2. transfer circuit
3. primary circuit (jets)
4. power valve
5. pump shot
any of those things can make it run rich, but each will do it at different rev ranges. the idle mixture will make it run rich at idle and have a slight affect on the primary circuit, the pump shot will have an affect on acceleration only, the power valve is for WOT when vacuum drops below whatever it is rated at, and jets run everything else.
so you really need to figure out where its running rich. lol. sorry that didnt help at all.
